1. Field of the Invention
This invention relates to a power transmission system for transmitting a driving force of a prime mover to driving wheels via a stepped transmission.
2. Description of the Prior Art
Conventionally, a power transmission system of the above-mentioned kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 2000-65199. The power transmission system includes a five-forward-speed transmission having first to fifth speed input gears arranged on an input shaft thereof which is connected to an engine, in the mentioned order from the engine side, and first to fifth speed output gears arranged on an output shaft thereof, in constant mesh with the respective first to fifth speed input gears. These first to fifth speed input and output gears form first to fifth speed gear pairs, respectively. Further, the first and second speed output gears are rotatably supported on the output shaft, and a synchromesh is arranged on the output shaft between the two gears. The synchromesh is selectively put into meshing engagement with and disengagement from one of the first and second speed output gears, to connect and disconnect the one of the output gears to and from the output shaft, whereby the speed position of the transmission is set to the first speed position or the second speed position. Similarly, the third and fourth speed input gears are rotatably supported on the input shaft, and another synchromesh is arranged on the input shaft between the two gears. The synchromesh selectively connects and disconnects one of the third and fourth speed input gears to the input shaft, whereby the speed position of the transmission is set to the third speed position or the fourth speed position.
On the other hand, the fifth speed input gear is rotatably supported on the input gear, and connected/disconnected to and from the input shaft by a shift clutch. The shift clutch is of a hydraulic type the engagement force of which is controllable. The shift clutch is arranged on an end portion of the input shaft on a side remote from the engine, and further, a shift cylinder for controlling the shift clutch is arranged at a location outward of the shift clutch. When the speed position of the transmission is set to the fifth speed position, the hydraulic pressure of the shift cylinder is maximized, whereby the shift clutch is completely engaged. On the other hand, in conducting up-shifting operations within a shift range up to the fourth speed by using the synchromeshes, the hydraulic pressure of the shift cylinder is reduced to a smaller value to allow the shift clutch to slide, whereby part of engine torque is transmitted and supplied to the output shaft as supplemental or compensating torque. In general, in torque transmission by a synchromesh alone, the transmitted torque is reduced to zero or a value close to zero during a time period from completion of synchronization of the synchromesh to establishment of complete meshing engagement thereof. The above control operation for the shift clutch is conducted to prevent this phenomenon of xe2x80x9ctorque transmission interruptionxe2x80x9d from giving the driver a feeling of inertia travel which is peculiar to the synchromesh.
However, in the conventional power transmission system, it is necessary to arrange the hydraulic shift clutch and the shift cylinder for actuating the same on the input shaft along the same in addition to the synchromeshes so as to prevent the driver from having the inertia travel feeling during an up-shifting operation, which increases the length of the transmission in a direction along the input shaft and hence makes it impossible to make the transmission and the power transmission system including the same compact in size.
It is an object of the invention to provide a power transmission system which is capable of positively preventing a feeling of inertia travel from being caused by torque transmission interruption which might occur during a time period between completion of synchronization of a synchromesh and establishment of meshing engagement of the same in an up-shifting operation, simply by using synchromeshes alone and without increasing the axial length of a transmission.
To attain the above object, the present invention provides a power transmission system for transmitting a driving force of a prime mover to driving wheels via a stepped transmission,
the stepped transmission comprising:
an input shaft connected to the prime mover;
an output shaft connected to the driving wheels;
a first gear pair for an L-th speed position, a second gear pair for an M-th speed position higher than the L-th speed position, and a third gear pair for a speed position other than and higher than the L-th speed position and the M-th speed position, each of the gear pairs having an input gear arranged on the input shaft, and an output gear arranged on the output shaft and in mesh with the input gear, with one gear of the input gear and the output gear being rotatably supported on the input shaft or the output shaft;
first to third synchromeshes for transmitting the driving force of the prime mover from the input shaft to the output shaft via the gear pairs; and
synchromesh control means for controlling respective operations of the first to third synchromeshes,
wherein when a speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the synchromesh control means causes the third synchromesh to move toward the third gear pair by a first predetermined amount which is not large enough to bring the third synchromesh into meshing engagement with the third gear pair, to thereby transmit the driving force of the input shaft to the output shaft.
According to this power transmission system, a first gear pair for an L-th speed position, a second gear pair for an M-th speed position, and a third gear pair for a speed position other than and higher than the L-th and M-th speed positions are formed by respective input gears arranged on the input shaft connected to the prime mover and respective output gears arranged on the output shaft and in mesh with the respective input gears, and there are provided first to third synchromeshes for transmitting the driving force of the prime mover from the input shaft to the output shaft via the gear pairs. When the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the synchromesh control means causes the third synchromesh to move toward the third gear pair by a first predetermined amount which is not large enough to bring the third synchromesh into meshing engagement with the third gear pair, to thereby transmit the driving force of the input shaft to the output shaft.
As described above, in the power transmission system, in the up-shifting operation for shifting the speed position of the transmission from the L-th speed position to the M-th speed position, the third synchromesh is moved toward the third gear pair by the first predetermined amount which is not large enough to bring the third synchromesh into meshing engagement with the third gear pair, whereby during this time period, the driving force of the input shaft is transmitted via the third gear pair to the output shaft as a supplemental (i.e. compensating) driving force. As a result, the second synchromesh can be put into complete meshing engagement under the condition of the supplemental driving force being thus supplied, which makes it possible to positively prevent the transmitted driving force (torque) from being reduced to zero or a value close to zero during the up-shifting operation, thereby preventing the operation of the synchromesh from causing the feeling of inertia travel peculiar thereto. Further, since the supplemental driving force can be supplied by utilizing the third gear pair and the third synchromesh for speed positions other than the ones directly involved in the up-shifting operation from the L-th speed position to the M-th speed position, it is unnecessary to add any special component parts so as to carry out the operation for supplying the supplemental driving force. Thus, it is possible to positively prevent the driver from having the inertia travel feeling, simply by using the existing synchromeshes alone and without increasing the axial length of the transmission.
Preferably, the first to third synchromeshes are each capable of performing disconnecting operation for disconnecting the one gear of the input gear and the output gear of a corresponding one of the first to third gear pairs from the input shaft or the output shaft rotatably supporting the one gear, baulking operation by the first predetermined amount of motion thereof, for power transmission until rotations of the one gear and the input shaft or the output shaft rotatably supporting the one gear are made synchronous, and connecting operation by a second predetermined amount of motion thereof, for connecting the one gear with the input shaft or the output shaft through the meshing engagement, and each for transmitting the driving force of the prime mover from the input shaft to the output shaft via a connected one of the gear pairs including the connected one gear, and when the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the synchromesh control means causes the second synchromesh to perform the baulking operation, and causes the third synchromesh to perform the baulking operation at least during a time period after the second gear pair is made synchronous by the baulking operation of the second synchromesh and before the synchromesh control means causes the second synchromesh to start the connecting operation.
According to the preferred embodiment, each synchromesh performs the disconnecting operation, the baulking operation, and the connecting operation for a corresponding one of the first to third gear pairs. When the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the third synchromesh is caused to perform the baulking operation at least during the time period from completion of the synchronization of the second gear pair by the baulking operation of the second synchromesh and the start of the connecting operation of the same, and hence during this time period, the driving force of the prime mover is transmitted from the input shaft to the output shaft via the third gear pair to supply a supplemental (i.e. compensating) driving force. As a result, the second synchromesh is put into complete meshing engagement through its connecting operation under the condition of the supplemental driving force being thus supplied, which makes it possible to positively prevent the transmitted driving force (torque) from being reduced to zero or a value close to zero during the time period from completion of the synchronization of the second synchromesh to the complete meshing engagement of the same, thereby preventing the operation of the synchromesh from causing the feeling of inertia travel peculiar thereto.
More preferably, when the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the synchromesh control means causes, on or after starting to cause the second synchromesh to perform the baulking operation, the third synchromesh to perform the baulking operation and the first synchromesh to perform the disconnecting operation, and then after completion of the synchronization of the second gear pair by the baulking operation of the second synchromesh, causes the second synchromesh to perform the connecting operation.
According to this preferred embodiment, in shifting the speed position of the transmission from the L-th speed position to the M-th speed position, the synchromesh control means causes, on or after starting to cause the second synchromesh to perform the baulking operation, the third synchromesh to perform the baulking operation and the first synchromesh to perform the disconnecting operation, and then after termination of the baulking operation of the second synchromesh, causes the second synchromesh to perform the connecting operation. Therefore, the driving force of the prime mover can be positively supplied to the output shaft via the third gear pair over the time period from the completion of the synchronization of the second synchromesh to the start of the connecting operation of the same, which makes it possible to positively prevent the driver from having the inertia travel feeling during execution of the up-shifting operation.
Further preferably, when the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the synchromesh control means starts to cause the third synchromesh to perform the baulking operation simultaneously with starting to cause the second synchromesh to perform the baulking operation.
According to this preferred embodiment, when the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the baulking operation is performed by the third synchromesh simultaneously with the baulking operation of the second synchromesh, and hence the driving force of the prime mover is supplied to the output shaft via the third gear pair as the supplemental driving force from the start of the up-shifting operation. Therefore, torque load applied to the second synchromesh can be reduced by the amount of the torque transmitted via the third synchromesh, which makes it possible to reduce the torque capacity of the second synchromesh and suppress wear of the same.
Preferably, the first gear pair and the third gear pair are arranged adjacent to each other, and the first and third synchromeshes are formed by a unitary synchromesh arranged between the first gear pair and the third gear pair.
According to this preferred embodiment, the operation for disconnection of the first gear pair for the up-shift and the operation for supply of the supplemental driving force via the third gear pair can be carried out by using the unitary synchromesh. This reduces the number of synchromeshes to be controlled, and hence makes it possible to simplify control operations for the up-shifting operation and the supply of the supplemental driving force.
More preferably, when the speed position of the transmission is shifted from the L-th speed position to the M-th speed position, the synchromesh control means causes the third synchromesh to perform the baulking operation, then causes the first synchromesh to perform the disconnecting operation and subsequently the second synchromesh to perform the baulking operation, and further after completion of the synchronization of the second gear pair by the baulking operation of the second synchromesh, causes the second synchromesh to perform the connecting operation.
According to this preferred embodiment, in shifting the speed position of the transmission from the L-th speed position to the M-th speed position, the synchromesh control means first causes the third synchromesh to perform the baulking operation, and then causes the first synchromesh to perform the disconnecting operation and subsequently the second synchromesh to perform the baulking operation. Further, after termination of the baulking operation of the second synchromesh, causes the second synchromesh to perform the connecting operation. Therefore, in this preferred embodiment as well, the driving force of the prime mover can be positively supplied via the third gear as the supplemental driving force over a time period from the completion of the synchronization of the second synchromesh to the start of the connecting operation of the same, which makes it possible to positively prevent the driver from having the inertia travel feeling during execution of the up-shifting operation. Further, since the second synchromesh is caused to perform the baulking operation after the disconnecting operation of the first synchromesh, the first and second gear pairs arranged adjacent to each other can be connected/disconnected by using the unitary synchromesh, and hence the present invention can be applied to a conventional transmission having a plurality of gear pairs arranged in order of speed positions, without changing the layout of the gear pairs.
Preferably, the M-th speed position is one speed position higher than the L-th speed position.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.